Heat treatment strategically strengthened door beam

ABSTRACT

A method of forming a vehicle component, particularly an elongated impact beam, having an open section structure, in a manner to provide predetermined strategically strengthened portions, comprising the steps of cold forming unhardened steel into a workpiece having mounting surfaces, selectively fixturing the mounting surfaces, static induction heating the workpiece with lengthwise surface eddy currents on selected portions, followed by quenching of the fixtured heated workpiece to form strengthened portions, and unfixturing the resulting component. Also disclosed is apparatus to accomplish this, and the resulting novel vehicle component.

BACKGROUND OF THE INVENTION

This invention relates to strategically strengthened steel automotivemembers of open elongated structure, and is particularly suitable forstrategically strengthened, open structure, automobile components, suchas impact door beams used for crash energy management.

Automotive vehicles employ crash energy management impact beams forprotection of passengers. A good share of these beams are of closedstructure type, e.g., cylindrical or tubular, as in U.S. Pat. Nos.5,370,437; 5,123,694; 5,118,159; and 4,708,390. Such tubular beams arenormally of generally uniform wall thickness. Further, they do not haveexposed edges because of their closed or tubular configuration.Therefore, they can be readily induction heat treated. These tubes canbe formed of lower hardness steel and then heat treated. Heat treatmentof such tubular impact beams can be achieved by induction simply byencircling successive portions of the beam with a heat treatmentinduction coil to heat the same, followed by quenching. Door beam tubesare sometimes made of special steel, as disclosed in U.S. Pat. No.4,210,467, and then cut to length and provided with a desired endconfiguration.

As an alternative to this type of tubular impact beam structure, it isknown to take flat steel, and form it into an open beam by cold stampingor rolling. There is a limit, however, to the hardness and strength ofthis type of final beam product, because the metal must not be so hardas to not be reliably formable by stamping or rolling. As anotheralternative, steel may be hot formed into the desired configuration, asdisclosed in U.S. Pat. No. 5,972,134. However, this latter alternativeis costly both in the per piece cost and the capital investmentrequired.

Prior efforts to form open section type impact beams from low strengthsteel, and then heat treat the beam, have resulted in at least twoimperfections which are not acceptable. First, if standard inductionheating with encircling coils is used, the free or exposed edges of thebeam tend to become overheated and burned, while the remainder of thecross section remains insufficiently heated. Second, the impact beamtends to become distorted as a result of the heating and subsequentquenching. Hence, such impact beams do not meet the required qualitystandards for easy and effective assembly in an automobile, or othervehicle. Furthermore, they do not have the required uniform strengthcharacteristics that are required in many applications due to the unevenheating.

It would therefore be desirable to fabricate automobile components suchas impact beams of open structure type from unhardened steel, andsubsequently harden selected portions of the steel without theimperfections and drawbacks previously experienced in the prior art.Such an open structure impact beam would preferably have elongated sideflanges along its length for added strength, but without the burnedflange edges and/or excessively distorted beam structure experienced inthe prior art, and with greater and more uniform strength.

SUMMARY OF THE INVENTION

One aspect of the present invention is a method of making an automobilecomponent, especially a vehicle impact beam, of the open structure type,and preferably with strengthening flanges, by cold forming it fromunhardened steel stock, and then induction heat treating the steelwithout burning or overheating the flanges, resulting in achieving ultrahigh tensile strengths of even up to about 238 ksi and yield strengthsof about 191 ksi, which is an increase of about 6 percent in the tensilestrength and about 15 percent in yield strength over those asserted forhot stamped beams. These are significant differences.

The impact beam is preferably formed of unhardened steel at ambienttemperature, with a pair of strengthening side flanges, preferablycurled, and a configured central area, preferably of hat-shapedcross-sectional configuration. The preferred hat-shaped area can be ofsingle or multiple hat cross section. The unhardened steel can bereadily cold formed by stamping or rolling as examples. The formedworkpiece is then hardened in strategically predetermined areas over itslength, by proximity coil induction heating while the beam is fixturerestrained, the induction process creating eddy currents running thelength of the selected areas of the beam rather than transversely of thelength, i.e., lengthwise along the crown, the crown sidewalls and/or theflanges.

These and other features, advantages and aspects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an impact door beam shown in combination withinner and outer portions of a lower proximity, usually static, inductioncoil;

FIG. 2 is a sectional view of the door beam in FIG. 1 in combinationwith an upper proximity, usually static, coil adjacent the crown of thedoor beam, inner proximity, usually static, lower coil turns adjacentthe sidewalls of the crown, and lower outer proximity, usually static,coil turns adjacent the flanges of the door beam;

FIG. 3 is a side elevational view of the door beam and the upper andlower induction coils;

FIG. 4 is a side elevational view of the door beam, induction coils andfixture mechanism at the ends of the door beam;

FIG. 5 is a side elevational view of the door beam and fixture within aquench subassembly;

FIG. 6 is an end elevational view of the door beam, induction coils andquenching subassembly;

FIGS. 7 and 7A-7D show some examples of configurations andstrengthening. Specifically:

FIG. 7 is a plan view of a final strategically strengthened door beamshown strengthened over its length except for the mounting end surfaces;

FIG. 7A is a plan view of a preferred embodiment of the strategicallystrengthened door beam, showing the strengthening portion to be onlycentrally of the door beam, encompassing the crown, the sidewalls andthe flanges;

FIG. 7B is a plan view of a door beam strategically strengthened in thecrown portion only;

FIG. 7C is a plan view of a similar door beam strategically strengthenedin the flange portions only; and

FIG. 7D illustrates a door beam strategically strengthened only in theportions of the crown straddling the central indentation or rib.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For purposes of description herein, the terms “upper”, “lower”, “right”,“left”, “rear”, “front”, “vertical”, “horizontal” and derivativesthereof shall relate to the invention as oriented in FIG. 1. However, itis to be understood that the invention may assume various alternativeorientations and step sequences, except where expressly specified to thecontrary. It is also to be understood that the specific devices andprocesses illustrated in the attached drawings, and described in thefollowing specification, are simply exemplary embodiments of theinventive concepts defined in the appended claims. Hence, specificdimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

Referring now specifically to the drawings, in FIGS. 1-4 is shown anautomobile component, namely an impact beam 10, which in the illustratedexample has a formed, cross-sectional structure or rigid body which isopen, i.e., not tubular or closed, shown having a crown 12 with anelongated indentation 14 centrally thereof to form a rib, a pair ofsidewalls 16 integral with the crown and also integral with a pair ofoutwardly extending flanges 18 which are preferably curled. Rib 14 andcurled flanges 18 add strength to the impact beam. While a single crownis depicted, the present invention may incorporate multiple crowns.Therefore, when the term “crown” is used herein, it is intended toencompass both a single crown and multiple crowns unless statedotherwise.

Adjacent to but spaced above crown 12 is shown an elongated, proximityinduction coil 20 having a lower turn and an upper turn, the lower turnbeing closely adjacent crown 12 but not in contact therewith, and theturns extending lengthwise of the crown and impact beam 10 over thedesired preselected portion of the crown 12 to be strategicallystrengthened. This arrangement causes induced eddy currents to move inthe primarily lengthwise axial direction of the workpiece.

It has been determined that the undesirable burned edge structure inprior art open channel impact beams is a result of the use of encirclinginduction coils for heat treatment. This is considered to be largelybecause the induced eddy currents at the surface of the open workpiecetransverse to the workpiece axis reversed at the lateral edges of theflanges, creating the undesirable edge overheating. This overheatingdoes not typically occur with enclosed or tubular types of impact beams,because the single direction transverse current flow on the closedsection is generally uniform, provided the thickness of the tubular beamis primarily uniform.

In the illustrated example, adjacent and inside the walls 16 of impactbeam 10 are shown multiple turns of an elongated proximity inductioncoil 24, wherein the turns extending lengthwise of the walls forinduction heating of sidewalls 16 by currents moving lengthwise in axialdirection of the workpiece. Adjacent to and straddling flanges 18 areturns of a lower, outer, elongated, proximity induction coil 28extending lengthwise of the flanges to cause induction heating by eddycurrents moving axially lengthwise of the flanges.

Each of the illustrated induction coils 20, 24 and 28 is connected to aconventional electrical power source (not shown). Also, each is normallyhollow for allowing coolant to flow through the coil. The turns of theinduction heater elements are normally ceramic coated for electricalprotection.

Preferably, conventional flux concentrators are utilized for optimumefficiency of the coils. Specifically, the upper surface of the lowerturn of coil 20 has a layer of flux concentrator. Also, an elongatedflux concentrator element 30 is positioned centrally of inner coil 24.Finally, the outer surfaces of coil 28 adjacent flanges 18 can have alayer of flux concentrator.

Coil 24 extends over the length of those portions of walls 16 which areto be strategically strengthened. Coil 28 extends over the length ofthose portions of the flanges 18 which are to be strategicallystrengthened. Since the impact beam 10 has a configuration which slopestoward the ends, such as in FIG. 3, upper coil 20 will have its lowerturn tapered in like fashion, so as to be closely adjacent to but out ofcontact with impact beam 10. Similarly, inner coil 24 will have aconfiguration generally matching that of walls 16.

The beam blank or workpiece is initially forcefully cold formed atsubstantially ambient temperature from non-hardened steel, such as bystamping and/or rolling techniques of conventional type, into thedesired configuration. A suitable material for the workpiece is ahardenable steel, i.e., quenchable steel. The two axial ends 11 can beformed into the flattened paddle shape shown in FIG. 1, or alternativelycan be a combination of paddle and flanged end, respectively, onopposite ends, or even a pair of flange ends, all of which are capableof serving as mounting surfaces. Preferably the beam is of one piece,but alternatively the ends can be formed separately and attached to themain portion of the impact beam.

In the illustrated example, prior to the heat treatment, the cold formedbeam workpiece is shown fixtured in a suitable clamping device,preferably by having both ends clamped securely in a fixture, as shownin FIG. 4. This elongated fixture 40 is shown to include a lower clampelement 40 a and an upper clamp element 40 b on one end. The upper clampelement 40 b may be rotatable on an axis to securely engage one end ofthe impact beam workpiece against the lower clamp element 40 a. Theopposite end of the fixture 40 includes a lower clamp element 40 c andan upper clamp element 40 d, wherein the upper clamp element 40 d isalso rotatable to engage the second end of the beam against the lowerclamp element 40 c. Preferably one pair of these clamping elements 40 aand 40 b and 40 c and 40 d allows the respective flange to move onlylongitudinally, but not vertically or torsionally, to accommodate beamexpansion and contraction due to temperature increases and decreasesduring the induction heat treating process, but prevent significantvertical or torsional distortion. Alternatively, both ends of thefixture 40 may allow longitudinal movement without permittingsignificant vertical or torsional movement. The fixture ends need not berotatable, but rather simply movable to restrain or release the impactbeam, e.g., normal to the beam axis. As a further alternative, thefixture 40 can clamp the center portions of the impact beam rather thanthe ends.

Once the impact beam workpiece is fixtured, electrical power is suppliedto the elongated coil or coils extending along the workpiece. Thedrawings show three coils, but these may alternatively be multiple turnsof one coil, two coils or some other number of coils. As here shown,upper coil 20, central coil 24 and/or lower outer coil 28 cause eddycurrents to be created lengthwise of the adjacent impact beam portions,and thereby significantly increase the temperature of the impact beamportions to a desired predetermined value and for a desiredpredetermined time to obtain appropriate metallurgical changes.Experimentation showed that use of a pulser to selectively interruptactivation of the coils creates pulse heating with lapsed time afterpulses for conduction disperses the heat uniformly, which isadvantageous in achieving uniform heat throughout the selected crosssection. Typical pulse/conduction heating is a 100 Kw, 30 Hz inductionpulse of about five seconds at 50 percent power, followed by a onesecond lapse for conductance, repeated as necessary until the targetsection is sufficiently heated. These power and time factors will varywith the specific item being treated. Preferably the length of all threeportions of the hat-shaped beam, i.e., crown 12, walls 16 and flanges18, is heated lengthwise thereof by eddy currents moving lengthwise ofthe beam. When the appropriate preselected temperature is reached forthe time necessary, the coils are deactivated, a pair of quench tankunits 50 a and 50 b of a quench subassembly 50 is moved adjacent to andastraddle of the door beam, and coolant is then rapidly and suddenlyapplied through nozzles 50 a′ and 50 b′ directed onto the impact beam 10to quench the same while it is still fixtured or retained in fixture 40,and thereby reduce the temperature of impact beam 10 to create thedesired hardening effect. If the induction coils interfere with thequenching apparatus, the coils may be moved before the quenching step.

The resulting impact beams 10 have significantly increased tensile andyield strengths compared to the known prior art hot stamped beams. Thisenables a lighter weight beam to be employed, thereby reducingmanufacturing costs and improving vehicle efficiency. It has been foundthat by utilizing the concept and method herein, ultra high strengths ofeven about 238 ksi tensile strength and about 191 ksi yield strength canbe achieved in the strategically strengthened portions of the openstructure impact beam. These are substantially above the 226 ksi tensilestrength and 167 ksi yield strength of the prior art hot stampedproducts. This is highly desirable.

FIGS. 7 and 7A-7D illustrate several possible single crown impact beamsstrengthened in selected zones as noted previously. The single crownconfiguration of each can be of multiple crown configuration.

The above description is considered that of the preferred embodimentsonly. Modifications of the invention will occur to those skilled in theart and to those who make or use the invention. Therefore, it isunderstood that the embodiments shown in the drawings and describedabove are merely for illustrative purposes and not intended to limit thescope of the invention, which is defined by the following claims asinterpreted according to the principles of patent law, including thedoctrine of equivalents.

1. An apparatus for making vehicle impact beams of the type having apredetermined open section configuration, comprising: means for coldforming an unhardened blank of hardenable steel at substantially ambienttemperature into an elongated vehicle impact beam having opposite ends,and an elongated open section therebetween having an offset lateralcross-sectional configuation with a center portion and a pair ofelongated flanges extending along opposite sides of the center portionwhich collectively define the predetermined open section configuration;an elongated induction heater having longitudinally extending coilspositioned adjacent to and extending along at least selected portions ofat least one of the center portion and the flanges of the vehicle impactbeam, which when activated, heats the selected portions of the vehicleimpact beam with eddy currents that move lengthwise along the surfacesof the selected portions of the vehicle impact beam to a temperaturesufficient to heat treat the hardenable steel of the vehicle impactbeam; a quencher rapidly cooling the heated selected portions of the oneof the center portion and the flanges of the vehicle impact beam toselectively harden and strengthen the same; and first and secondclamping fixtures spaced longitudinally apart adjacent the opposite endsof the vehicle impact beam, and including first and second clampingmembers which converge to securely retain therebetween the opposite endsof the vehicle impact beam and prevent the vehicle impact beam fromshifting either torsionally or laterally from said predetermined opensection configuration during activation of said induction heater andsaid quencher; at least one of said first and second clamping fixturesbeing configured to permit at least one of the clamped ends of thevehicle impact beam to shift longitudinally between said first andsecond clamping members, while preventing torsional and lateral shiftingof the vehicle impact beam to accommodate for thermal expansion andcontraction of the vehicle impact beam during activation of saidinduction heater and said quencher.
 2. An apparatus as set forth inclaim 1, wherein: said induction heater includes a first enlongatedinduction heater coil disposed adjacent to and between interior surfacesof the center portion and flanges of the vehicle impact beam, andextending along a predetermined portion thereof.
 3. An apparatus as setforth in claim 2, wherein: said induction heater portion includes asecond elongated induction heater coil disposed adjacent an exteriorsurface of the center portion of the vehicle impact beam, and extendingalong a predetermined portion thereof.
 4. An apparatus as set forth inclaim 3, wherein: said induction heater includes a third elongatedinduction heater coil disposed adjacent an exterior surface of a firstone of the flanges, and extending along a predetermined portion thereof.5. An apparatus as set forth in claim 4, wherein: said induction heaterincludes a fourth elongated induction heater coil disposed adjacent anexterior surface of a second one of the flanges, and extending along apredetermined portion thereof.
 6. An apparatus as set forth in claim 5,wherein: said first elongated induction heater coil includes first andsecond laterally spaced apart portions; and including a concentratorelement positioned between the first and second portions of said firstenlongated induction heater coil.
 7. An apparatus as set forth in claim6, wherein: said first elongated induction heater coil is taperedlongitudinally to remain closely adjacent to but out of contact with thevehicle impact beam.
 8. An apparatus as set forth in claim 7, wherein:said induction heater includes a pulser to create substantially uniformheating of the selected portions of the vehicle impact beam.
 9. Anapparatus as set forth in claim 1, wherein: said induction heaterincludes an elongated induction heater coil disposed adjacent to andbetween interior surfaces of the center portion and flanges of thevehicle impact beam, and extending along a predetermined portionthereof.
 10. An apparatus as set forth in claim 1, wherein: saidinduction heater includes an induction heater coil disposed adjacent anexterior surface of the center portion of the vehicle impact beam, andextending along a predetermined portion thereof.
 11. An apparatus as setforth in claim 1, wherein: said induction heater includes an inductionheater coil disposed adjacent an exterior surface of a first one of theflanges, and extending along a predetermined portion thereof.
 12. Anapparatus as set forth in claim 1, wherein: said induction heaterincludes an induction heater coil disposed adjacent an exterior surfaceof a second one of the flanges, and extending along a predeterminedportion thereof.
 13. An apparatus as set forth in claim 1, wherein: saidlongitudinally extending coils are tapered longitudinally to remainclosely adjacent to but out of contact with the vehicle impact beam. 14.An apparatus as set forth in claim 1, wherein: said induction heaterincludes a pulser to create substantially uniform heating of theselected portions of the vehicle impact beam.
 15. An apparatus as setforth in claim 1, wherein: said first and second clamping fixtures areeach configured to permit the clamped ends of the vehicle impact beam toshift longitudinally between said first and second clamping members,while preventing torsional and lateral shifting of the vehicle impactbeam to accommodate for thermal expansion and contraction of the vehicleimpact beam during activation of said induction heater and saidquencher.